In particular, this invention relates to that class of electrolytic capacitors called "computer type" capacitors which are intended for mounting on printed circuit boards of sophisticated electronic devices such as computers. The main feature of such capacitors is a life requirement of ten years or more at normal use temperatures which may range as high as 85.degree. C.
The life of a capacitor is appreciably shortened when solvent vapor and/or electrolyte fluid escape past terminals in the header. Further, a capacitor which leaks electrolyte is undesirable because the electrolyte may attack the printed circuit board to which the capacitor is connected or provide a conductive path between the runs on the printed circuit board.
The standard construction for over twenty five years employed a plastic header with two molded-in, threaded aluminum terminals of the type shown in Collins et al (U.S. Pat. No. 3,789,502) with the plastic being molded around the terminals generally as shown in Pearce et al (U.S. Pat. No. 4,074,417).
The standard construction, in addition, does not lend itself to soldered connections to the copper runs on the ordinary printed circuit board in common use. Often a mounting bracket is required and external connections must be made by means of wires with terminal lugs which attach to the capacitor terminals by screws with lock washers.
Zeppieri (U.S. Pat. No. 3,398,333) and Schroeder (U.S. Pat. No. 4,183,600) both teach prior art capacitors in which an aluminum serrated shank terminal extends through a thermal plastic header. In both these patents, the aluminum terminal is resistance heated to a temperature such that the length of the terminal is collapsed and the center diameter increases to press the serrations into the melted plastic. However, the terminal necessarily has low strength when heated, as evidenced by the fact that the very low pressure of 75 psi is specified in Schroeder in order to expand the diameter. It is therefore obvious that only an extremely light interference fit can be developed between the terminal and the header. Schroeder further refers to the problem of thermal cycling as requiring a gasket under the enlarged end of the terminal as improvement on Zeppieri. This indicates that the interference fit is inadequate and adequate sealing of the terminal cannot be obtained without an additional gasket.
Another requirement for computer type capacitors is that they be immune to de-fluxing agents used in the cleaning and/or manufacture of the printed circuit boards which have been wave soldered. For example, residual amounts of a chlorinated solvent, such as methylene chloride, which remain on the capacitor after processing the printed circuit board, can attack and destroy aluminum terminals which must operate at anodic potential. It is the standard practice in the aluminum electrolytic capacitor industry to place protective epoxy over capacitor terminals if exposure is expected. Such protection is costly and takes up space.
An example of such a solvent is Dupont Freon.RTM. TMC (disclosed in U.S. Pat. No. 2,999,817) which is used to clean fluxes from printed circuit boards. Nonepoxy protected capacitors when soaked at 25.degree. C. in Freon TMC for four hours and then submitted to a normal life test, fail before completion of the life test.